1. Field of the Invention
The present invention relates to a method for washing a semiconductor substrate having a silicon oxide film formed thereon, and a washing apparatus therefor.
2. Discussion of Background
Semiconductor devices are formed through many production steps. Between the respective steps, if necessary, removal of dust particles or contaminants adhered to the surface of a semiconductor substrate is carried out, or during the etching, a treatment using acid type or alkali type chemicals is carried out. Subsequently, washing is carried out with pure water containing no chemicals. The washing is carried out for the removal of chemicals, since if chemicals remain on the surface of the semiconductor substrate, adverse affects are given to the properties of semiconductor devices.
For example, if hydrochloric acid remains on the surface of a semiconductor substrate, when a metallic wiring of aluminum or copper is formed thereon, there are problems that chlorine of the hydrochloric acid reacts with the metal to cause corrosion and breakage of the wiring. Accordingly, after the treatment with chemicals, it is required to finally conduct washing with pure water containing no chemicals.
However, by the washing with pure water for the removal of the chemicals, silicon oxides are formed on hydrophobic portions of the surface of the semiconductor substrate. Further, it has been known that during the progressing from a certain step to the next step, an oxide film is naturally formed by exposing the semiconductor substrate to air. Along the progress toward the fine structure of semiconductor devices, such a silicon oxide (film) largely influences the film properties of its upper and lower adjoining films, by which electrical properties and reliability of semiconductor devices are reduced
To remove such a silicon oxide film, it has been attempted to immerse the semiconductor substrate in pure water containing hydrofluoric acid or pure water containing ammonium fluoride and hydrofluoric acid, and then wash it with pure water containing no chemicals. By the washing with the pure water containing no chemicals, fluorine atoms bonded to the silicon atoms of the surface of the semiconductor substrate are replaced by hydrogen atoms. Such a washing is essential since a film further formed thereon tends to be irregular without it.
FIGS. 8 and 9 show conventional washing apparatuses of semiconductor substrates. In the Figures, 101 is a treatment bath, 102 is a semiconductor substrate, 103 is a supply line, 104 is a supply valve and 105 is a cassette. To remove a silicon oxide (film) formed on the surface of a semiconductor substrate, pure water containing hydrofluoric acid or pure water containing hydrofluoric acid and ammonium fluoride, and pure water containing no chemicals, are used, for each of which an exclusive washing apparatus is provided.
For example, when an silicon oxide (film) is removed with pure water containing hydrofluoric acid, firstly, a treatment bath 101 is filled with pure water containing hydrofluoric acid by means of a supply line 103 as shown in FIG. 8, and then a semiconductor substrate 102 having a silicon oxide (film) formed on its surface is immersed to remove the silicon oxide (film).
Then, into a treatment bath 101 of another washing apparatus, pure water containing no chemicals is supplied by means of a supply line 103, and then a semiconductor substrate 102 is immersed in the treatment bath 101 to remove the hydrofluoric acid.
In such washing apparatuses, both the pure water containing hydrofluoric acid and the pure water containing no chemicals are supplied by means of each supply line 103. The pure water containing hydrofluoric acid is circulated and used several times, whereas the pure water containing no chemicals is overflowed from the treatment bath 101.
The semiconductor substrate 102 may be put in the treatment bath 101 singly or in groups of plural pieces. Further, when plural pieces are treated all at once, the semiconductor substrates 102 may be put in a cassette 105 as shown in FIG. 9.
However, after the removal of the silicon oxide (film), when washing is conducted with pure water containing no chemicals to remove the hydrofluoric acid or ammonium fluoride adhered to the surface of the semiconductor substrate, there are problems such that a silicon oxide (film) is formed again by the below-mentioned reactions.
Firstly, by the treatment of the silicon oxide (film) adhered to the surface of the semiconductor substrate with hydrofluoric acid, SiF62xe2x88x92 ions are formed on and in the vicinity of the surface of the semiconductor substrate. The SiF62xe2x88x92 ions are formed during the etching of the silicon oxide film with substances containing fluorine.
Then, pure water containing no chemicals is supplied to initiate a washing treatment, whereupon the pH value is rapidly raised and a hydrolysis reaction of the SiF62xe2x88x92 ions is caused as shown in the following formula (1).
SiF62xe2x88x926H2O⇄Si(OH)62xe2x88x92+6HFxe2x80x83xe2x80x83(1)
The Si(OH)62xe2x88x92 ions formed by this reaction undergo a dehydration condensation reaction to one another, whereby colloidal silica consisting of SiO2 is formed in the solution and adhere to the surface of the semiconductor substrate. The colloidal silica is a silicon oxide in a colloid state, and causes a problem such that the colloidal silica adheres to the surface of the semiconductor substrate made of silicon and becomes a silicon oxide (film), thereby deteriorating the properties of semiconductor devices.
Further, especially when a semiconductor device is formed by using a semiconductor substrate having a silicon oxide film of about 3,000 xc3x85 formed on its back, by treating plural pieces of semiconductor substrates put in a cassette 105 all at once as shown in FIG. 9, there are problems such that more colloidal silica may be generated from the back of the semiconductor substrate 102 than its surface, and resultingly more silicon oxide (film) will be formed on the surface of the adjoining semiconductor substrate.
Under such circumstances, the present invention have been made. It is an object of the present invention to provide a method for washing a semiconductor substrate wherein after the removal of a silicon oxide (film) formed on the surface of the semiconductor substrate, no silicon oxide (film) is formed again even if washed with pure water containing no chemicals, by which the properties of semiconductor devices can be improved, and a washing apparatus therefor.
The present invention provides a method for washing a semiconductor substrate having an oxide formed on its surface , which comprises a step for immersing the semiconductor substrate in a first treatment bath to which hydrofluoric acid and pure water are supplied; a step for transporting the semiconductor substrate from the first treatment bath to a second treatment bath to which an inorganic acid and pure water are supplied to adjust the pH value to not more than 7 and immersing the semiconductor substrate in th e second treatment bath; and a step for supplying only pure water to the second treatment bath and washing the semiconductor substrate.
Preferably, after supplying the inorganic acid and the pure water to the second treatment bath, the concentration of the inorganic acid supplied to the second treatment bath is reduced stepwise so that only water is supplied.
Further, the inorganic acid is preferably hydrochloric acid.
The present invention also provides a washing apparatus for washing a semiconductor substrate which has an oxide formed on its surface and is treated with hydrofluoric acid, which comprises a treatment bath to which an inorganic acid and pure water are supplied for washing the semiconductor substrate; a first supply line for supplying the pure water which has a supply valve and is connected to the bottom of the treatment bath; and a second supply line for supplying the inorganic acid which has a supply amount-controlling means and is connected to the first supply line.
The present invention further provides a washing apparatus for washing a semiconductor substrate having an oxide formed on its surface, which comprises a treatment bath to which hydrofluoric acid, an inorganic acid and pure water are supplied for washing the semiconductor substrate; a first supply line for supplying the pure water which has a supply valve and is connected to the bottom of the treatment bath; a second supply line for supplying the inorganic acid which has a first supply amount-controlling means and is connected to the first supply line; and a third supply line for supplying the hydrofluoric acid which has a second supply amount-controlling means and is connected to the first supply line.
Further, the supply amount-controlling means is preferably a valve.